I would insist upon a bore-scope inspection of any older rifle chambered for a magnum cartridge like the .300 Weatherby, 7mm Remington, etc..
What happens is that in many cases, the outside of the rifle looks pristine but the part that you cannot see, the throat area of the barrel will have heat-cracking or be worn out. This kind of damage is invisible if you are looking down the bore with a light. Only a bore-scope will tell the tale.
If it looks like this - keep looking.
If they balk at the idea of an inspection by a gunsmith, chances are that they have something to hide.
Reputable online sellers offer a 30-day inspection period. Inspecting the rifle visually before accepting it at the FFL holder is always a good idea. Have arrangements with a gunsmith for a bore-scope inspection after you take it home, so if it is fried inside you can return it right away and get your money back.
I have been stuck this way with a 7mm Rem mag, and a .243 that looked almost new on the outside but would not shoot accurately. A bore-scope inspection showed why. The .243 is not a magnum, but it is an over-bore cartridge like a magnum, easy to fry inside if shot while hot.
On the other hand, a lot of .300 Weatherbys are sold by people who have shot them one or maybe two times, then put them in the safe and never shot them again. There are a lot of good buys out there in used Mk V's.